OLED devices have drawn great attention in display industries, and particularly in the flat-panel display industry, because it is operable with a low driving voltage and capable of generating light of red, green, and blue colors with high luminance efficiency. These unique attributes are derived from a basic OLED structure comprising of a multilayer stack of thin films of small-molecule organic materials sandwiched between an anode and a cathode. Typically, OLED devices are fabricated in forward-stacked structures with the anode placed in contact with the substrate or support on which the OLED is constructed, and the cathode placed on the top of the OLED.
Low power consumption displays typically use an active matrix backplane where thin film transistors (TFT made of a-Si or LTPS) drive current to the OLED. In this case, the OLED stack is located at the source of the TFT, thus the anode of the OLED pixel is directly connected to the source of the driving TFT. Although this manufacturing process is much simpler, the circuit becomes dependent on the characteristics of the OLED materials. Any changes in the OLED voltage due to its aging behavior will affect both the voltage Vg between the gate and the source and the current Ids flowing through the driving TFT and OLED pixel.
Alternately, in an inverted OLED configuration, the deposition of the organic layers is reversed, where the cathode of the OLED is deposited first in order to connect the OLED cathode to the drain of the driving TFT. Thus, if the OLED stack can be located at the drain of the drive TFT, changes in the OLED characteristics affects only the current Ids and not the voltage Vg between gate and source. Thus, the inverted OLED configuration is more compatible to the active-matrix organic light emitting display (AMOLED) to increase the design variability of the active-matrix driving circuit and raise the efficiency of the AMOLED.
In manufacturing of the inverted OLED, the key factors are the charge transported characteristic and the charge injection capability on the interfaces between device electrode and organic material, and between organic material and another organic material. In particular, a critical problem with the inverted OLED exists in that voltage rise over time can be very large. Various designs of the inverted OLED have been proposed to solve the problems, such as using materials of lower work function as the cathodes of the OLED. However, with the constant use and operation of the OLED devices, deterioration of the materials of the organic layers would decrease the intensity of electroluminescence of the OLED device. This creates difficulties in achieving desired luminance.
Moreover, in color inverted OLED devices, materials for emitting lights in different colors would have different degrees of deterioration within the same operation time. Thus, the lifetime of the electroluminescent units of the inverted OLED devices would be different, which results in color shift after long operation time of the OLED devices.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.